Subsea installations, particularly oil or well installations, are often connected with pipe which is laid along the sea floor for the purpose of conducting fluid such as gas, oil, and water from a subsea installation to another installation or to an onshore installation or to an offshore platform or vessel. Such pipe may be of different diameter and in length may amount to thousands of feet. It is apparent that because of the environment in which the pipe must be laid, many problems are encountered.
Prior proposed pipe laying arrangements have included stacking a supply of straight pipe lengths on a barge and connecting as by welding the ends of the straight pipe lengths as required during the pipe laying operation. Such intermittent step-by-step laying of pipe is time consuming and costly.
To obviate the disadvantages of welding ends of straight pipe together on a vessel at sea for providing a continuous pipe, other prior proposed constructions included the winding of pipe on a drum of a reel. The reel was mounted on a barge or vessel and the pipe lengths were welded together prior to being wound upon the reel. In this system the drum of the reel was made very strong and heavy to carry the pipe. As pipe was drawn onto the reel by rotation of the reel the pipe was coiled in the form of a cylinder about the drum for the length of the drum and then a second cylindrical form of coil turns was wound upon the first coil. Since the pipe was wound under tension and such forces acted at surface areas of the pipe caused by bending and interfaced contact with adjacent pipe coils, protective coatings applied to the pipe would be damaged, marred, or broken. A pipe reel containing pipe cylindrically coiled about its drum was extremely heavy and difficult to manipulate. Moreover, the winding operation was preferably performed on the vessel because the wound pipe reel was too heavy to handle or required special derrick equipment in order to transfer such a pipe-filled reel from a dock or pier to a vessel.
Examples of prior proposed constructions showing pipe wound upon a reel are British Pat. No. 601,103 and U.S. Pat. No. 3,685,306 in which the reel axis is horizontal and U.S. Pat. Nos. 3,237,438, 3,372,461, 3,712,100 and 3,630,461 in which the reel axis is vertical. In such prior constructions the path of pipe from the point of departure from the reel to the straightening means varies horizontally or vertically with each coil turn and is continually changing in the direction of approach to the straightening means over a relatively wide range, that is the length of the cylindrical form.
Prior proposed pipe laying constructions of these types presented difficulties and limitations in handling because of the structural strength required, transfer of the pipe fitted reel from one place to another, and difficulties encountered in withdrawing a pipe from a cylindrical form and in laying pipe of different diameter.